In addition, its longer-term use was not associated with increases in blood pressure or increased risk of heart attack, stroke or death, results showed, published in journal Obesity.

"Although diet and exercise are critical components of any weight-loss programme, up to half of patients don't have long-term success with lifestyle changes alone," said Kristina H. Lewis, Assistant Professor at the Wake Forest School of Medicine in the US.

"In those cases, medications or surgery can help. Generic phentermine is an effective and affordable option, but now that we view obesity as a chronic disease, it's important to have medications that can be used indefinitely. Most new weight-loss drugs are approved for long-term use, but unfortunately the newer drugs can be expensive if they are not covered by insurance."

However, Lewis cautioned that phentermine is a stimulant and should not be used in people with a history of heart disease, stroke or uncontrolled high blood pressure.

But, it could be used by those with low cardiac risk, normal blood pressure or high blood pressure that is well treated.

More clinical trials to provide further certainty are needed, said the team.

For the study, the researchers included 13,972 adults who were prescribed phentermine for short-term use versus longer-term use of a year or more.